November 4, 2010 1 Comment
Timeline, 2008: Sexual selection is a mechanism of evolution that sometimes butts heads with natural selection. Under the tenets of natural selection, nature chooses based on characteristics that confer a competitive edge in a given environment. Under this construct, environment is “the decider.” But in sexual selection, either competition between the same sex or a choice made by the opposite sex determines the traits that persist. Sometimes, such traits aren’t so useful when it comes to the everyday ho-hum activities like foraging for food or avoiding predators, but they can be quite successful at catching the eye of an interested female.
Those female opinions have long been considered unchanging. In the widowbird, for example, having long, flowing black tailfeathers is a great way to attract the lady widow birds. But perhaps they don’t call them widowbirds for nothing: if those male tailfeathers get too long, the bird can’t escape easily from predators and ends up a meal instead of a mate. In these cases, natural selection pushes the tailfeather trait in one direction—shorter—while sexual selection urges it the other way—longer. The upshot is a middling area for tailfeathers length.
This kind of intersexual selection occurs throughout the animal kingdom. Probably the most well-recognized pair that engages in it is the peacock and peahen. Everyone has seen the multicolored baggage any peacock worth his plumage drags around behind him. A peacock will fan out those feathers in an impressive demonstration, strutting back and forth and waving its tail in the wind, showing off for all he’s worth. It’s a successful tactic as long as nothing is around that wants to eat him.
Frogs hoping for a mate find themselves elbow deep in the “paradox of the lek.” The lek is the breeding roundup for frogs, where they all assemble in a sort of amphibian prom. For the males, it’s a tough call, literally. They must call loudly enough to show the females how beautifully androgenized they are—androgens determine the power of their larynx—while at the same time not standing out enough to attract one of the many predators inevitably drawn to a gathering of hundreds of croaking frogs. Trapped in this paradox, the frog does his best, but natural selection and sexual selection again end up stabilizing the trait within expected grooves.
This status quo has become the expectation for many biologists who study sexual selection: natural selection may alter its choices with a shifting environment, but what’s hot to the females stays hot, environmental changes notwithstanding. But the biologists had never taken a close look at the lark bunting.
A male lark bunting has a few traits that may attract females: when it shakes off its drab winter plumage and takes on the glossy black of mating season, the male bird also sports white patches on its wings that flash through the sky and sings a song intended to draw in the ladies. But the ladies appear to be slaves to fashion, not consistently choosing large patches over small, or large bodies over lighter ones. Instead, female lark buntings change their choices with the seasons, selecting a large male one year, a dark-colored male with little in the way of patches the next, and a small-bodied male the next. Lark buntings select a new mate each year, and the choice appears to be linked to how well the male will aid in parenting duties, which both parents share. It may be that a big body is useful in a year of many predators, but a small body might work out better when food supplies are low.
The researchers who uncovered this secret of lark bunting female fickleness watched the birds for five years and based their findings on statistical correlations only. For this reason, they don’t know exactly what drives the females’ annually varying choices, but they speculate that environmental factors play a role. Thus, sexual selection steps away from the realm of the static and becomes more like—possibly almost indistinguishable from—natural selection.